The invention relates to an apparatus and method for pumping a liquid from a well. The apparatus and method is especially intended for pumping oil from oil wells, although it may be used for pumping water from water wells and for other similar purposes.
The most common type of pump used for pumping oil from wells comprises a pump unit disposed at the bottom of the well, a drive unit or pump jack disposed adjacent the top of the well and a sucker rod connecting a reciprocating arm on the pump jack to a piston in the pump unit. Reciprocation of the sucker rod by the pump jack causes the piston of the pump unit to pump oil up the well. Such apparatus has the disadvantage of employing a sucker rod equal in length to the depth of the well and of imposing varying stresses on this sucker rod at various phases of the pumping cycle. During its upstroke, the upper end of the sucker rod is not only bearing the whole weight of the sucker rod but also the weight of the column of liquid extending the whole way up the well which is being lifted by the pump unit. Even in wells of comparatively modest diameter the load on the sucker rod may amount to about 1.6 pounds per foot (2.38 kg/m.) of well depth and thus about 16,000 lbs. (7250 kg.) in a 10,000 foot (3048 m.) deep well. These large and rapidly varying stresses tend to cause frequent breakages of the sucker rods in conventional oil well pumps incorporating rocker arm pump jacks and such breakages not only result in loss of production from the well but also require the expensive services of a skilled crew and a large amount of equipment to fish the broken portion of the sucker rod from the well in order that the sucker rod may be replaced and production from the well resumed.
To avoid the problems associated with breakage of such sucker rods, it is known to use a form of oil well pump havng no sucker rod but instead having a well-bottom pump unit comprising a drive cylinder and a driven cylinder, each of these cylinders having a piston slideable therein, the two pistons being interconnected to that movement of the drive piston will cause movement of the driven piston. The drive piston is hydraulically driven by fluid supplied from a hydraulic pump in the drive unit or pump jack at the top of the well via a line extending between the pump jack and the well-bottom pump unit, while reciprocation of the driven piston within the driven cylinder causes oil to be pumped up a liquid discharge line. The return hydraulic fluid from the drive cylinder may return to the surface in either the liquid discharge line or via a separate return line extending up the well.
One major problem which is encountered in such hydraulically-driven well-bottom pumps is arranging for the change of direction of the drive piston as it reaches either end of the drive cylinder. Because of the presence of grit and sand at the well bottom, the well-bottom pump unit is required to operate in very dirty conditions and must be very reliable because oil well pumps are left to operate for at least several days at a time unattended and because any failure of the well-bottom pump involves a time consuming and expensive removal of the well-bottom pump unit to the top of the well for repair or replacment of the pump unit. Furthermore, in order to achieve maximum production from the well, it is desirable that the pump be double-acting and thus the drive piston must be driven in both directions. Finally, in some cases it may be desirable to control the movement of the drive piston so as not to allow this piston to go right to the ends of the drive cylinder at the extremities of its motion nor allow this piston to remain stationary at the end of its stroke for any appreciable time. Sudden decelerations imposed upon the drive piston as this piston comes into contact with an end wall of the drive cylinder as the drive piston reaches either extremity of its motion may cause undesirable oscillations and pressure surges (coning effects) in the oil surrounding the pump unit at the bottom of the well. Moreover, pauses in the motion of the drive piston may result in loss of production from the well.
Conventional well-bottom pump units reverse the direction of the flow by valve means associated with the drive cylinder and actuated by the movement of the drive piston along the drive cylinder. Many types of valve means have been devised for this purpose, most of them being extremely complicated, sophisticated and expensive to produce in order that they can survive the extremely rigorous conditions under which they must operate at the bottom of a well. However, the more complicated the valve means employed, the more susceptible it is to mechanical failure, and any such mechanical failure of a valve means associated with the pump unit at the bottom of the well necessitates the lengthy and costly removal of the pump unit from the well for repair or replacement of the valve means.
It will be appreciated that the disadvantages mentioned above are not confined to oil wells, but may be experienced in other wells, such as water wells, which draw liquid from strata surrounding the well and pump it to the surface in subtantially the same manner as an oil well.
There is thus a need for a hydraulically-operated well pump which is extremely reliable in operation and which avoids the use of complicated valves which are likely to fail under the rigorous operating conditions experienced in oil and other wells. Furthermore, it is desirable that the pump allow its valve means to be located on the surface adjacent the well so that the valve means is readily accessible for repair or replacement. The invention provides such an apparatus.